The present invention relates to gas detecting and warning systems of the type employing a metal oxide gas sensor device. More particularly, the invention relates to a new and improved gas detecting and warning system of such type which incorporates a monolitic quad comparator integrated circuit means to provide a system which is extremely stable and reliable in operation, produces a distinctive failure indication and warning upon failure of the gas sensor device and/or associated circuitry to function, and one which is easier and less expensive to manufacture and calibrate.
Metal oxide gas sensor devices are commercially available and have been employed in various prior types of gas detecting and warning systems, such as for example, those systems shown and described in the following U.S. Pat. Nos.:
3,631,436--Taguchi;
3,733,595--Benedict;
3,860,919--Aker;
3,895,367--Visser.
Gas sensor devices of the metal oxide type must be heated to a predetermined temperature since the resistance between the electrodes is very small when the semiconductor sensing element is at room temperature and gradually increases to a very high value. After a predetermined period, usually in the range of one to three minutes or so, the device attains a stabilized condition at which it exhibits maximum sensitivity. When the device is then exposed to a contaminating gas, such as carbon monoxide, hydrogen, hydrocarbons, or the like, the resistance between the electrodes is significantly reduced and the current flow through the device is considerably increased. Many prior art systems have utilized this change in current flow directly to control the operation of active devices such as transistors. Such arrangements, although convenient, leave much to be desired since the operating characteristic tolerances vary over a broad range which creates difficulty both from the standpoint of reliability of operation of the system, as well as, from the standpoint of manufacture and calibration.
Further, since during the foregoing stabilizing heating period the system is prone to produce false warnings, it is desirable that the warning alarm means be inhibited for a predetermined period of time after the system has been energized.
Also, since such systems are relied upon to always be ready to function when necessary and produce a warning in the presence of the contaminating gas, it is desirable that the system be fail-safe and provide an indication upon the failure of the gas sensor device to function. All of the foregoing are provided in the gas detecting and warning system of the present invention and in a very simple, inexpensive and reliable manner.
Accordingly, it is an object of this invention to provide a new and improved gas detecting and warning system employing a gas detector device of the metal oxide type which substantially overcomes one or more of the prior art difficulties and in an arrangement which is less complex, less expensive to manufacture and more reliable than any other known prior art system.
It is another object of this invention to provide a new and improved gas detecting and warning system employing a gas detector of the metal oxide type and which employs a monolithic quad comparator integrated circuit means in a novel arrangement to achieve stable and reliable comparison between a "safe" and an "unsafe" ambient condition, stable and reliable time delay, simple sesitivity adjustment, and a "fail safe" failure detection and warning.
It is a further object of the invention to provide a gas detecting and warning system employing a gas detector of the metal oxide type which can be operated from an A-C or D-C power source as well as from an A-C power source with D-C stand-by power.